Abstract
Abstract. Climate models have traditionally only represented heat and water fluxes within relatively shallow soil layers, but there is increasing interest in the possible role of heat and water exchanges with the deeper subsurface. Here, we integrate an idealized 50 m deep aquifer into the land surface module of the GISS ModelE general circulation model to test the influence of aquifer–soil moisture and heat exchanges on climate variables. We evaluate the impact on the modeled climate of aquifer–soil heat and water fluxes separately, as well as in combination. The addition of the aquifer to ModelE has limited impact on annual-mean climate, with little change in global mean land temperature, precipitation, or evaporation. The seasonal amplitude of deep soil temperature is strongly damped by the soil–aquifer heat flux. This not only improves the model representation of permafrost area but propagates to the surface, resulting in an increase in the seasonal amplitude of surface air temperature of > 1 K in the Arctic. The soil–aquifer water and heat fluxes both slightly decrease interannual variability in soil moisture and in land-surface temperature, and decrease the soil moisture memory of the land surface on seasonal to annual timescales. The results of this experiment suggest that deepening the modeled land surface, compared to modeling only a shallower soil column with a no-flux bottom boundary condition, has limited impact on mean climate but does affect seasonality and interannual persistence.
Highlights
The land-surface components of global climate models (GCMs) have typically represented water and heat storage as occurring only in a surface layer 1–5 m deep (Manabe, 1969; Koster and Suarez, 1996; Milly and Shmakin, 2002; Essery et al, 2003; Dickinson et al, 2006; Schmidt et al, 2006)
Heat exchange beneath the soil column affects the seasonal cycle of soil temperature and its response time to global warming, which is important for modeling permafrost processes (Nicolsky et al, 2007; Alexeev et al, 2007)
A one-layer aquifer will somewhat overstate seasonal soil–aquifer heat fluxes compared to a model with finer vertical discretization (Alexeev et al, 2007), so that the effect we find on climate of the soil–aquifer heat flux can be regarded as an upper limit of what we would see as we make our subsurface model more realistic
Summary
The land-surface components of global climate models (GCMs) have typically represented water and heat storage as occurring only in a surface layer 1–5 m deep (Manabe, 1969; Koster and Suarez, 1996; Milly and Shmakin, 2002; Essery et al, 2003; Dickinson et al, 2006; Schmidt et al, 2006). Heat exchange beneath the soil column affects the seasonal cycle of soil temperature and its response time to global warming, which is important for modeling permafrost processes (Nicolsky et al, 2007; Alexeev et al, 2007). In view of these considerations, a number of studies in recent years have coupled models of deep water and heat storages with models of land-surface and atmosphere processes. Regional model studies by Anyah et al (2008) and by Jiang et al (2009) both found that groundwater storage and flow tends to increase available soil moisture and warm-season evaporation rates in the semiarid
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